1. Technical Field
This disclosure relates generally to thin-film magnetoresistive read sensors and particularly to the enhancement of the magnetoresistive properties of such sensors by the insertion of additional layers.
2. Description
In simplest form, the usual giant magnetoresistive (GMR) read sensor consists of two magnetic layers, formed vertically above each other in a parallel planar configuration and separated by a conducting, but non-magnetic, spacer layer. Each magnetic layer is given a unidirectional magnetic moment within its plane and the relative orientations of the two planar magnetic moments determines the electrical resistance that is experienced by a current that passes from magnetic layer to magnetic layer through the spacer layer. The physical basis for the GMR effect is the fact that the conduction electrons are spin polarized by interaction with the magnetic moments of the magnetized layers. This polarization, in turn, affects their scattering properties within the layers and, consequently, results in changes in the resistance of the layered configuration. In effect, the configuration is a variable resistor that is controlled by the angle between the magnetizations.
The magnetic tunneling junction device (TMR device) is an alternative form of GMR sensor in which the relative orientation of the magnetic moments in the upper and lower magnetized layers controls the flow of spin-polarized electrons tunneling through a very thin dielectric layer (the tunneling barrier layer) formed between those magnetized layers. When injected electrons pass through the upper layer, as in the GMR device, they are spin polarized by interaction with the magnetization direction (direction of its magnetic moment) of that layer. The probability of such an electron then tunneling through the intervening tunneling barrier layer into the lower magnetic layer then depends on the availability of states within the lower layer that the tunneling electron can occupy. This number, in turn, depends on the magnetization direction of the lower layer. The tunneling probability is thereby spin dependent and the magnitude of the current (tunneling probability multiplied by the number of electrons impinging on the barrier layer) depends upon the relative orientation of the magnetizations of magnetic layers above and below the barrier layer.
In what is called a spin-valve configuration, one of the two magnetic layers in both the GMR and TMR has its magnetization fixed in spatial direction (the pinned layer), while the other layer (the free layer) has its magnetization free to move in response to an external magnetic stimulus. If the magnetization of the free layer is allowed to move continuously, as when it is acted on by a continuously varying external magnetic field, the GMR and TMR device each effectively acts as a variable resistor and it can be used as a read-head in a hard disk drive. If the magnetization of the free layer is only permitted to take on two orientations, parallel and antiparallel to that of the pinned layer, then the device can be used to store information (eg. 0 or 1, corresponding to the free layer magnetization orientation) as an MRAM cell.
The difference in operation between the GMR sensor and the TMR sensor, is that the resistance variations in the former are a direct result of changes in the electrical resistance (due to spin polarized electron scattering) within the three-layer configuration (magnetic layer/non-magnetic conducting layer/magnetic layer), whereas in the TMR sensor, the amount of current is controlled by the availability of states for electrons that tunnel through the dielectric barrier layer that is formed between the free and pinned layers.
When the TMR configuration is used as a sensor or read head, (called a TMR read head, or “tunneling magnetoresistive” read head) the free layer magnetization is required to move about a central bias position by the influence of the external magnetic fields of a recorded medium, such as is produced by a moving hard disk or tape. As the free layer magnetization varies in direction, a sense current passing between the upper and lower electrodes and tunneling through the dielectric barrier layer varies in magnitude as more or less electron states become available. Thus a varying voltage appears across the electrodes (which may be the magnetic layers themselves). This voltage, in turn, is interpreted by external circuitry and converted into a representation of the information stored in the medium.
A typical bottom spin valve GMR sensor structure is the following: Seed/AFM/outer pinned (AP2)/Ru/inner pinned (AP1)/Cu/Free Layer/Capping Layer.
A typical bottom spin valve TMR sensor structure is the following: Seed/AFM/outer pinned (AP2)/Ru/inner pinned AP1)/MgO/Free Layer/Capping Layer,
In the TMR configuration shown above (and in the CPP GMR as well), the seed layer is an underlayer required to form subsequent high quality magnetic layers. The AFM (antiferromagnetic layer) is required to pin the pinned layer, ie., to fix the direction of its magnetic moment by exchange coupling. The pinned layer itself is now most often a synthetic antiferromagnetic (SyAF) (also termed a synthetic antiparallel (SyAP)) structure with zero total magnetic moment. This structure is achieved by forming an antiferromagnetically coupled tri-layer whose configuration is denoted herein as “outer pinned (AP2)/Ru/inner pinned (AP1)”, which is to say that two ferromagnetic layers, the outer (farthest from the free layer) and inner (closest to the free layer) pinned layers which are denoted AP2 and AP1 respectively, are magnetically coupled across a Ru spacer layer in such a way that their respective magnetic moments are mutually antiparallel and substantially cancel each other. The structure and function of such SyAP structures is well known in the art and will not be discussed in further detail herein.
In the GMR sensor (i.e., used as a read head) there is an electrically conducting but non-magnetic spacer layer (typically of Cu) that separates the free and pinned layers. This conducting, but non-magnetic Cu spacer layer in the GMR is replaced in the TMR by a thin insulating (dielectric) layer of (for example) oxidized magnesium (MgO) that can be oxidized in any of several different ways to produce an effective dielectric tunneling barrier layer. The free layer in both the GMR and TMR is usually a bilayer of ferromagnetic material such as CoFeB/NiFe, and the capping layer in both the GMR and TMR is typically a layer of tantalum (Ta). The bilayer choice for the free layer is strongly suggested by the fact that an effective free layer should be magnetically soft (of low coercivity), which is an attribute of the CoFeB layer. The CoFeB layer, however, exhibits excessive magnetostriction. By adding the NiFe layer, the magnetostriction is reduced, but unfortunately, the TMR ratio, dR/R, (ratio of maximum resistance variation as the free layer magnetic moment changes direction, dR, to total device resistance, R), which is a measure of its efficacy as a read sensor (or MRAM element), will also be reduced. We shall see below that the structure of the free layer can be significantly altered to provide an improved TMR sensor or MRAM element as well as a GMR sensor or MRAM element. We note that the vertical positioning of the free and pinned layers may be reversed, to form either what are called “bottom spin valves” (as shown here) and, alternatively “top spin valves” with the free layer formed on the seed layer and the pinned layer vertically above the free layer.
Much recent experimentation on GMR sensors has been directed at improvements in the free layer structure. The most common structure in both the GMR and TMR sensor had been a CoFeB/NiFe bilayer, in which the NiFe layer provides the low magnetostriction, while the CoFeB provides good magnetic softness. More recently, attempts have been made to improve the magnetic properties of both free and pinned layers by utilizing novel materials and laminated structures. Examples of such attempts, which differ from and do not achieve the results of the present disclosure, are to be found in:    U.S. Pat. No. 7,672,088 (Zhang et al), which is assigned to the present assignee.    U.S. Pat. No. 8,008,231 (Nishimura et al.)    U.S. Pat. No. 8,085,511 (Yuasa et al.)